Surface Flow Outlets to Protect Juvenile Salmonids Passing Through Hydropower Dams

Abstract

We reviewed results of research conducted by engineers and biologists over the past 50 years related to development of surface flow outlets (SFOs) for juvenile salmonids that migrate downstream past hydropower dams. An SFO is a nonturbine, water-efficient passage route with an overflow structure through which flow and fish pass over a dam. Our review covered 69 SFOs in Europe and North America. We identified five main types of SFOs—low-flow bypass/sluices, high-flow sluices, forebay collectors, powerhouse retrofits, and surface spills. Most low-flow bypass/sluices are sited in Europe and on the east coast of North America, where mean annual project discharge and power production were 95 m 3 /s and 15 MW, respectively. The other four SFO types are found on the west coast of North America with 2184 m 3 /s mean annual discharge and 788 MW mean power output. A conceptual framework based on fish behavior and hydraulics for different regions of a hydropower project was developed to evaluate SFO performance. Based on available data, fish collection efficiency averaged 53%, with an average effectiveness ratio of 17:1 (percentage of fish collected and passed to percentage of inflow used for SFO attraction). Surface flow outlet technology can meet the goal of concurrent anadromous fish protection and hydropower generation.